STUDIES ON CHROMOSOMES. 225 



generations of germ cells which lie inward toward the lumen. 

 As in other well known vertebrates the spermatozoa attach 

 themselves to a Sertoli or nurse-cell for a period before their 

 complete maturation and ejection from the tubule. 



The usual four types of cells, (i) spermatogonia, (2) primary 

 spermatocytes, (3) secondary spermatocytes, and (4) spermatids, 

 are present. After a period of spermatogonial divisions, various 

 of the spermatogonia enlarge to become primary spermatocytes 

 which divdde to form secondary spermatocytes. The latter 

 divide again to form the spermatids which ultimately transform 

 into spermatozoa. The spermatocytes, both primary and 

 secondary, and the spermatids, seem to be unattached, or at 

 most, to be very loosely attached in the tubule and they there- 

 fore readily spill out onto a slide when the tubules are cut. 

 Consequently it is comparatively easy to get an abundance of 

 these cells for smears. On the other hand, it is very difficult 

 to secure spermatogonia in smears in sufficient numbers for 

 purposes of study. They adhere firmly to the tubule wall and 

 even after mincing with scalpel or scissors are seldom found in 

 any considerable quantity. 



As seems general in cases in which the germinal cells are 

 arranged in seminiferous tubules, there appear to be proliferating 

 and resting zones in the same tubule, or possibly some entire 

 tubules are quiescent while others are active. This is evinced by 

 the fact that sections or smears from certain regions of the testis 

 show no active mitoses while others exhibit them in varying 

 degrees of abundance. In still other preparations, mainly sper- 

 matids, or spiremes of primary spermatocytes, or some other 

 characteristic stage, constitute the main part of the preparation, 

 as if that special part of the testis were in a particular phase of a 

 general wave of spermatogenesis. 



A generalized scheme of the spermatogenesis is shown in 

 Text-figure I. The spermatogonial chromosomes (a, b), repre- 

 sented in the diagram as sixteen straight, rod-like chromosomes 

 and two curved chromosomes, divide equationally to produce 

 two sets (c) of daughter chromosomes. Details regarding the 

 curved chromosomes are given in later pages. Before the next 

 division, which is that of the primary spermatocyte, the usual 



